Phosphofructokinase has been studied in Tetrahymena pyriformis. The level of this enzyme in glucose-grown organisms is greater than that in cells grown without glucose. Starvation of glucose-grown cells leads to a very rapid loss of PFK activity, indicating an enzyme-mediated reaction. In crude cell-free extracts, enzyme activity is stabilized by ATP. Mg ions and Ca ions decrease phosphofructokinase activity, whereas, Na ion increases activity. Attempts to purify phosphofructokinase from this organism resulted in enzyme activity associated with large yet unidentified particles.